Ontogenetic and environmental responses in metabolic enzyme activity of Pacific Arctic larval gadids

Warming in high-latitude marine ecosystems is leading to the borealization of Arctic communities. Species-specific responses to temperature provide insight into potential co-occurrence or competitive advantage between Arctic and boreal species. Ocean acidification may also lead to unique species-specific responses. At the Pacific-Arctic interface, larval distributions of the boreal Pacific cod (<i>Gadus macrocephalus</i>) are increasingly overlapping with those of Arctic cod (<i>Boreogadus saida</i>). We assessed larval metabolic capacities by measuring metabolic enzyme activities of citrate synthase (CS; aerobic metabolism), lactate dehydrogenase (LDH; anaerobic metabolism), and β-hydroxyacyl CoA dehydrogenase (HOAD; fatty acid metabolism). Throughout early development, Pacific cod enzyme activities, including glycolytic capacity, were higher, and fatty acid metabolism lower than Arctic cod enzyme activities. These responses may reflect a more active larval lifestyle of Pacific cod. Separately, larvae were reared in multiple temperatures (Pacific cod: 3, 6, 10°C; Arctic cod 1.8, 5, 7.3°C) and pCO₂ levels (ambient = ~350 μatm; high = ~1500 μatm). At the cold temperature, Pacific cod enzyme activities were higher than at the control temperature, indicating they were acclimating but less cold adapted than Arctic cod. Arctic cod HOAD activity and LDH:CS ratio were elevated under warmer temperatures suggesting increased energy demand. Elevated pCO₂ levels only affected larvae at their control temperature and resulted in decreased Pacific cod HOAD activity and increased Arctic cod CS and HOAD activities. This indicates differing sensitivities to ocean acidification between the species. Overall, Pacific cod may continue to be constrained in their northern habitat by cold temperatures, but under slight warming to optimal growing temperatures, Pacific cod will have competitive advantage over Arctic cod.